1. Field of the Invention
The invention relates generally to holographic data storage methods and systems, and more particularly to methods and systems for holographic data storage recovery by oversampling and fractional-delay filter interpolation.
2. Description of the Related Art
Holographic data storage systems store information or data based on the concept of a signal beam interfering with a reference beam at a holographic storage medium. The interference of the signal beam and the reference beam creates a holographic representation, i.e., a hologram, of data elements as a pattern of varying refractive index and/or absorption imprinted in a volume of a storage or recording medium such as a photopolymer or photorefractive crystal. A spatial light modulator (SLM), for example, can create the data-encoded signal beam. The interference pattern induces material alterations in the storage medium that generate the hologram. The formation of the hologram in the storage medium is a function of the relative amplitudes and polarization states of, and phase differences between, the signal beam and the reference beam. The hologram is also dependent on the wavelengths and angles at which the signal beam and the reference beam are projected into the storage medium. After a hologram is created in the storage medium, projecting the reference beam into the storage medium interacts and reconstructs the original data-encoded signal beam. The reconstructed signal beam may be detected by using a detector, such as CMOS photo-detector array or the like. The recovered data may then be decoded by the photo-detector array into the original encoded data.
In typical holographic data storage systems it is important to align the SLM, detector (i.e., a camera), and the data storage medium such that each pixel of the SLM is projected onto a single pixel of the detector. This alignment is desired for a single hologram or a group of holograms stored by various multiplexing methods including angle, shift, wavelength, correlation, spatial, aperture, phase code, and the like. Aligning the pixels of the SLM, stored holographic image, and detector is commonly referred to as “pixel matching.” One objective of pixel matching is to obtain recovered images of data-containing holograms on the detector that have a low number of bits decoded in error in relation to the total bits of the data page, i.e., a low bit error rate (BER).
Performance of a holographic storage system, i.e., the quality of the modulated image, therefore depends at least in part on the alignment of various components such as the SLM with various devices, such as light sources, lenses, detectors, the storage medium, and the like. Generally the position and alignment of the SLM and other device components for reading and writing to the storage medium are mechanically set at the time of manufacturing the holographic storage system. Over time, however, the SLM, detector, or storage medium may become misaligned with various other components of the particular system. For example, temperature change, vibration, shock, and the like may result in slight movements or deformations of the detector, SLM, storage medium, or other optical components. The result may be translational, tilt, or rotational misalignment of the detector with the storage medium or the SLM. Further, in systems with removable storage media, such as a rotating disk or rectangular storage media, the media may be misaligned when inserted into one or more systems for read and/or write operations.
The desired alignment of components in a holographic storage system in the x and y direction is typically on the order of a few microns or less, and the rotation better than approximately 0.001 degree. In addition, the magnification of the system from drive to drive is desirably maintained to a high degree (typically better than 0.01%). The desired magnification generally results in the effective focal length of the lens between different drives to match on the order of tens of micron level. Such matching is difficult to achieve in low cost high volume processes. For example, these constraints on pixel matching may result in a holographic drive system generally having a higher cost, larger size, with increased error rates, and lower storage capacity than otherwise possible.
One strategy proposed for correcting errors arising from pixel misalignment due to improperly positioned system components and the like is to apply image-processing techniques to the recovered hologram image. An example of such a method is described in “Compensation for Pixel Misregistration in Volume Holographic Data Storage,” by G. W. Burr and T. Weiss published in Optics Letters, Vol. 26, No. 8, Apr. 15, 2001, the entire contents of which is incorporated herein by reference. Another approach is to adjust or tilt the reference beam to realign the image onto the detector during a read out process. Such a method is described in U.S. Pat. No. 5,982,513 entitled “Method and System to Align Holographic Images,” the entire content of which is incorporated herein by reference.
Deficiencies of these and other various methods include, for example, that system parameters are generally known or determined very precisely. Some image processing techniques are computationally intensive, may require repeated iterations to converge, suffer from error propagation, and may require channel transfer characteristics, i.e., point spread functions as it varies over the image. Additionally, there is no soft decision information as used by some error correction codes, for example, turbo codes and Low Density Parity Check (LDPC) codes.
New methods and devices are therefore needed for reliably adjusting for pixel misalignments and recovering holographic stored data with reduced error rates. In particular, methods and systems for processing detected holographic images with misalignments are needed.